Expandable retractable structure

ABSTRACT

An expandable-retractable structure which in its simplest form comprises: a first set of three rigid links connected together by a first primary pivot at one end, a second set of three rigid links connected together by a second primary pivot at one end and pivotally joined to the first set of links by means of three secondary pivots each connecting corresponding links of said two sets, and a set of three bracing cables connecting the three secondary pivots to complete a double tetragonal structure. The structure is usable as a single module or in combination with a plurality of similar modules which, when combined with connecting cables, may form flat, tapered, or curved structures such as bridges, towers, outriggers, and scaffolds. An actuation system is provided comprising an extension cable connecting the primary pivots which when subjected to tension urges said first and second primary pivots toward one another, subjecting the bracing cables to tension and thereby stabilizing the structure, and a retraction means comprising cables connecting the three secondary pivots which when subjected to tension urge the secondary pivots toward one another and the primary pivots away from one another to return the structure to its original retracted form.

United States Patent [191 Vaughan Nov. 13, 1973 EXPANDABLE RETRACTABLESTRUCTURE Desmond H. Vaughan, San Diego, Calif.

[73] Assignee: General Dynamics Corporation, San

Diego, Calif.

[22] Filed: May 30, 1972 [21] Appl. No.: 257,904

[75] Inventor:

Primary Examiner-John E. Murtagh Attorney-John R. Duncan et al.

[57] ABSTRACT An expandable-retractable structure which in its simplestform comprises: a first set of three rigid links connected together by afirst primary pivot at one end, a second set of three rigid linksconnected together by a second primary pivot at one end and pivotallyjoined to the first set of links by means of three secondary pivots eachconnecting corresponding links of said two sets, and a set of threebracing cables connecting the three secondary pivots to complete adouble tetragonal structure. The structure is usable as a single moduleor in combination with a plurality of similar modules which, whencombined with connecting cables, may form flat, tapered, or curvedstructures such as bridges, towers, Outriggers, and scaffolds. Anactuation system is provided comprising an extension cable connectingthe primary pivots which when subjected to tension urges said first andsecond primary pivots toward one another, subjecting the bracing cablesto tension and thereby stabilizing the structure, and a retraction meanscomprising cables connecting the three secondary pivots which whensubjected to tension urge the secondary pivots toward one another andthe primary pivots away from one another to return the structure to itsoriginal retracted form.

9 Claims, 17 Drawing Figures PATENIEUNUV13 191s sum 3 OF 6 FIG. 6 v

PATENTEDNUY13 1973- 3.771. 274

SHEET u [If e PAIENIEUNBY 13 ms 3; 771; 274

VAVA AYA AYAY i gw EXPANDABLE RETRACTABLE STRUCTURE BACKGROUND OF THEINVENTION The invention relates to a structural system which may beemployed in a wide variety of structures, including but not necessarilylimited to bridges, towers, outriggers, booms, scaffolds and structuralbeams, wherever a quickly erectable structure is required. The inventionfurther relates to a structural actuation system which willautomatically expand or retract the structure.

A need has long existed for support structures having a high degree ofportability in terms of compactness and light weight. When civildisaster strikes, it is usually required to quickly erect structures,such as communication towers or temporary bridges as examples, whichpossess the characteristic of portability. The structure must be capableof being moved into an area and set up for operation as quickly aspossible. Further, it must be capable of being quickly collapsed orcompressed into a compact package for rapid movement into other areas asdirected by the flow or character of the disaster.

Heretofore, such features have been difficult to achieve, and structureswere erected by assembling the structure in-place utilizing elementshaving quick attachment features or by assemblying prefabricatedstructural modules, which often required that a temporary falsework befirst erected to support the workers and the partially completed primarystructure. Modular-type construction, hinged rigid panels, andprefabricated standard structural elements all have the disadvantage ofbeing bulky and cumbersome since, for reasons of interchangeability, theelements are sized to carry the peak load that may occur anywhere in thestructure, but only occurs in a minority of the actual assembledstructure. Secondly, most such structures require, to varying degrees,that assembly must be completed by workers at the site.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an improved expandable-retractable structure which overcomes theabove mentioned difficulties.

Another object of the invention is to provide a fully fabricatedexpandable-retractable load supporting truss structure that is foldableinto a compact unit for storage or portage purposes.

Another object of the invention is the provision of a light weightstructure which is capable of being erected in a short time with aminimum of manpower, erection tools, or other facilities and structures.

Another object of the invention is the provision of a fully fabricatedmodular expandable structure which is easily portable and which whenerected provides a strong and sturdy load carrying structure.

Another object of the invention is to erect a structure without the needof additional accessories and materials which after accomplishing therequired purpose may be easily retracted into a compact portable packagefor subsequent use in the erection of another structure.

A further object of this invention is the provision of an actuationsystem for a truss structure which is capable of fully erecting thestructure remotely on command without the use of hand tools or theutilization of manpower.

Another object of the invention is the provision of an actuation systemfor a portable truss structure which is capable of remotely retractingthe structure into a compact portable package on command without the useof hand tools or the utilization of manpower.

Another object of the invention is the provision of a structure whichmay be expanded from a compact package into a straight structure, or ifdesired it may be configured to expand into a curved structure havingsingle or compound curvature.

Another object of the invention is to provide an actuation system forexpanding or retracting the structure whereby the elements of theactuation system comprise a portion of the load carrying structure whenit is expanded.

The above objects and others are accomplished by the present inventionutilizing a novel combination of flexible and rigid link structuralelements interconnected to provide a compactly stored package which willexpand into a rigid, light weight, load supporting structure. Thestructure comprises a plurality of modules wherein an individual modulecomprises two tripods, each rigid link of the first tripod beingpivotally connected to a corresponding link of the second tripod to formthree secondary pivots, and three flexible links or cables connectingthe three secondary pivots one to the other and thereby limiting theexpansion of the two tripods to form a double tetragonal structure.Contiguous modules are pivotally connected to one another at theirrespective secondary pivots and by cables at their respective primarypivots, said cables becoming taut when the structure is in the fullyexpanded position. The shape of the structure when expanded is dependentupon the individual lengths of the link elements, and since the entirestructure is preassembled, there is no danger of assembling an incorrectlength element in the structure at the erection site, as is possiblewith unassembled erectable structural elements currently in use.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the presentinvention reside in the construction and cooperation of elements ashereinafter described, reference being made to the accompnaying drawingsforming a part of this disclosure wherein:

FIG. 1 shows a basic embodiment of the structural module, comprising twotripods pivotally joined together at each end of each rigid link.

FIG. 2 is a top view of the module in the retracted position.

FIG. 3 shows an integrated structural arrangement comprising a pluralityof structural modules placed side by side.

FIG. 4 is an enlarged view of the upper primary pivot.

FIG. 5 is an enlarged view of a secondary pivot.

FIG. 6 is an enlarged view of the lower primary pivot.

FIG. 7 shows an embodiment which includes an actuator means locatedwithin the structural module of FIG. 1 for expanding the structure.

FIG. 8 shows an embodiment which includes a retracting means in themodule of FIG. 7.

FIG. 9 is an enlarged partially sectioned view of the actuator tube ofFIG. 8.

FIG. 10 is a top view of the module of FIG. 8 in the retracted position.

FIG. 11 is an enlarged cross-section of the actuator tube of FIG. 9showing an embodiment which includes a non-reversing slider locatedtherein.

FIG. 12 shows the module of FIG. 8, wherein the actuator tube islengthened in such a manner that the tube may be utilized as one of thestructural elements.

FIG. 13 is an enlarged partially-sectioned view of the actuator tube ofFIG. 12.

FIG. 14 is an enlarged cross-section of the actuator tube latch taken online 1414 in FIG. 12.

FIG. 15 is a diagrammatic plan view of a double curvature structurecomprising a plurality of modules having various length flexible links.

FIG. 16 is a diagrammatic cross-section of the curved structure taken online l6-16 in FIG. 15.

FIG. 17 is a cross-section view of the curved structure taken on line1717 in FIG. 15 showing one individual structural module.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings indetail, FIG. 1 is a view of the expanded structural module in itssimplest form. The module when expanded may be described as two tripods,one comprising a first set of rigid links 4, 5 and 6, and the secondcomprising a second set of rigid links 7, 8 and 9. Links 4, 5, and 6 arepivotally joined together at a first primary pivot l, and links 7, 8,and 9 are pivotally joined together at a second primary pivot 2. Each ofthe rigid links of the first tripod are joined to a corresponding rigidlink of the second tripod, so that link 4 is pivotally joined to link 7at a secondary pivot'3, and link 5 is joined to link 8 at anothersecondary pivot 3, and links 6 and 9 are pivotally joined at anothersecondary pivot 3. The rigid links 4, 5, 6, 7, 8, and 9 may be square,rectangular, channel, or round in cross-section, and may be constructedfrom any suitable material such as plastic or metal. It has been foundadvantageous to construct these members of aluminum or stainless steelalloy round tubing. Three bracing flexible links, 10, 11, and 12 areconnected to the three secondary pivots such that bracing link 10 isconnected from rigid links 4 and 7 to rigid links 5 and 8, bracing link11 connects rigid links 5 and 8 to rigid links 6 and 9, and bracing link12 connects rigid links 6 and 9 to rigid links 4 and 7. The flexiblelinks 10, 11 and 12 may be constructed from any suitable material suchas straps, single or multiple strand rope, chain, or cable of anysuitable material such as plastic or metal. It has been foundadvantageous to construct these members of aluminum or stainless steelalloy cable to which terminal fittings may be joined by swaging, and forconvenience these flexible links will hereinafter be simply referred toas cables. From the preceding it will be seen that the module shown inFIG. 1, with primary pivots 1 and 2 constrained or loaded toward eachother, is a stable shape with bracing cables 10, 11, and 12 assumingtension and thereby maintaining the module shape.

When primary pivots 1 and 2 are moved apart, the bracing cables 10, 11and 12 relax and the three secondary pivots 3 move together until therigid links, 4 thru 9, are parallel to one another. This geometricarrangement thus possesses the ability to move either to an expandedposition as illustrated in FIG. 1 or to a slim retracted position, whichwhen viewed from above, as shown in FIG. 2, displaces an approximatelyequilateral triangular volume, which is naturally adaptable tohexagonal, rectangular, or triangular shaped storage packages when aplurality of the retracted modules are packaged together.

FIG. 3 illustrates a plurality of deployed structural modules, of thetype illustrated in FIG. I, placed side by side in a common plane andconstrained in the open or expanded configuration. The primary pivots land 2 are each spanned by an assembly of cables and 200, respectively,and contiguous modules are pivotally joined one to another at theirrespective secondary pivots 3, with bracing cables 10, 11, and 12 ofeach module forming a third assembly of cables 300 thus forming a largeintegrated structure possessing the same expandable and retractablecapabilities as the basic structural module shown in FIG. 1. When themodules have been expanded until their respective bracing cables 10, 11,and 12 have become taut and primary pivot cable assemblies 100 and 200have become taut, the resulting expanded structure is of a fullytriangulated stable geometry having all the useful rigid structurecharacteristics associated therewith, and capable of use as a structuralbeam in bridges or towers, as examples. Three modules are particularlycalled out in FIG. 3 and are identified by the location of theirrespective primary pivots, 1 and 2, as position A, position B, andposition C. It will be noted that at position A a total of six cables,101 thru 106, of top cable assembly 100 are connected to primary pivot1, and six lower assembly cables, 201 thru 206, are connected to primarypivot 2. In the embodiment shown in FIG. 3, six top and six bottomcables are the maximum number connected to any individual primary pivot1 or 2.

The module located at position B is along the edge of the structurewhere the upper primary pivot l is connected by four upper cables, 102thru 105, and the lower primary pivot 2 is connected also by four lowercables, 202 thru 205, to the contiguous modules. Further, it should benoted that two of the three secondary pivots 3 of the position B modulewhich are located along the edge of the structure are each pivotallyattached to four rigid links, and the third pivot 3 is attached to sixrigid links.

The module located at position C forms a corner of the structure wherethree upper cables, 102 thru 104,

located in the corner connects two links, the pivot 3 located on theedge connects four links together and the third pivot 3 located insidethe structure connects six rigid links together.-

Referring to FIGS. 4, 5, and 6, which are enlarged views showing detailsof an upper primary pivot l, a secondary pivot 3, and a lower primarypivot 2, respectively, itwill be noted that certain structural membersshown in FIG. 3 have been eliminated to more clearly show the details ofthe individual pivots. All three pivots, 1, 2, and 3 comprise aplurality of tongues circumferentially located in three groups of threetongues each, wherein the center tongue of each group is adapted topivotally attach to a rigid link, and the tongues located to either sideare each adapted for attachment of a cable. Specifically, in FIG. 4, theupper primary pivot 1 comprises three tongues 21, 22 and 23 groupedtogether with the center tongue 22 provided with a hole 26, adapted topivotally attach rigid link 5 thereto, and tongues 21 and 23 are eachprovided with a hole 28 adapted to attach upper cables 100, thereto. Ina similar manner two additional groups of tongues 21, 22, and 23 arelocated around the circumference of pivot 1 for attachment of rigidlinks 4 and 6, and other upper cables 100. Any suitable means forattaching rigid links and cables may be utilized such as clevis pins,bolts, or rivets.

The secondary pivot 3 of FIG. 5 is provided with three tongues 31, 32,and 33 grouped together with the center tongue 32 provided with twoholes 36 for attachment of upper link 5 and lower link 8 thereto.Tongues 31 and 33 are each provided with a hole 38 for attachment ofbracing cables and 11 thereto. In a similar manner two other groups oftongues 31, 32, and 33 are located on the secondary pivot 3 forattachment of upper and lower rigid links 4, 7 and 6, 9 of adjacentmodules. It will be noted that the center tongues 32 each contain athird hole 40, which is not utilized in the structure illustrated inFIG. 3. The function of the third hole 40 will be later describedherein.

The lower primary pivot 2, of FIG. 6 is likewise provided with threegroups of tongues each group containing three tongues 41, 42, and 43,and again the center tongue 42 is adapted to pivotally attach rigidlinks 7, 8, and 9 thereto, while each of the other tongues 41 and 43 areprovided for attachment of lower cables 200, thereto.

From the foregoing description it will have been observed that primarypivots l and 2 serve as pivoting means for each of their respectivethree rigid links, totalling the six rigid links, 4 thru 9, of anindividual module, whereas each secondary pivot 3 serves as the pivotingmeans for only two rigid links of any one individual module, but mayprovide this pivoting means for up to three separate modulessimultaneously.

FIG. 7 shows an embodiment which includes an expanding actuator locatedwithin the basic structural module, wherein the actuator comprisestension actuator spring 50 connected at one end to the first primarypivot l and connected at the other end to an actuator cable 51. Actuatorcable 51 is connected to the second primary pivot 2, thereby providing atension tie between primary pivots 1 and 2 and biasing these primarypivots toward each other to cause expansion of the module into a stablestructural form. Maintenance of the stable structural form is dependenton either sustained tension in the extension cable 51 or externalcompression loads at pivots l and 2 producing a net tension in bracingcables 10, 11 and 12 under all loading conditions of the expandedstructure. It should be understood that the actuator spring 50 may spanthe entire distance between the two primary pivots 1 and 2 therebyeliminating the actuator cable 51, or conversely the actuator cable maybe constructed of an elastic material, such as bungee or shock cord, forexample, and span the entire distance between the primary pivots 1 and 2for biasing the primary pivots toward one another.

FIGS. 8 and 9 show an embodiment which includes a retracting actuator inthe module of FIG. 7, wherein the actuator comprises an actuator tube 52attached to pivot l, and retracting cables 54, 56, and 58 each attachedto actuator spring 50 by means of a cable connector 59 and running downtube 52 to the lower end of tube 52 and then to each of the threesecondary pivots 3 where connection is made to tongue 32 (FIG. 5)

by means of a clevis pin, or equivalent, utilizing the mounting hole 40(FIG. 5) located in tongue 32. It will be noted that the actuator tube52 is approximately the same length as the first set of rigid links 4,5, and 6 so that in the retracted position the three secondary pivots 3are in close proximity to the lower end of the actuator tube 52, therebyaffording good mechanical advantage to the retracting cables 54, 56, and58 as they draw the three secondary pivots 3 together. To retract themodule from the expanded position, actuator cable 51 is disconnectedeither from primary pivot 2 or from actuator spring 50, the preferredbeing to disconnect from pivot 2 so that the actuator cable 51 is drawninto actuator tube 52 by the actuator spring 50 for storage. Anysuitable cable disconnect may be used such as for example the embodimentshown in FIG. 14 which will be later described herein. At the same timethat extension cable 51 is disconnected, the three retracting cables 54,56 and 58 are being drawn up into the actuator tube 52 by actuatorspring 50, causing the three secondary pivots 3 to move together andprimary pivot 2, now disconnected from cable 51, to freely move downwardand farther away from primary pivot 1, and the geometry of the structurereturns to its original retracted position. Thus it can be seen that theinitial upward travel of spring 50 moves primary pivot 2 upward, bymeans of cable 51, until all slack is removed from bracing cables 10,11, and 12 whereupon the module is in the stable expanded position, andwhen actuator cable 51 is subsequently disconnected from primary pivot2, the second portion of upward travel of the spring 50 will causeretraction, as previously described. A plurality of such actuatorsystems operated simultaneously, one in each of the structural modules,causes general deployment and general retraction of the total structuralassembly.

FIG. 10 is a top view of the structural module of FIG. 8 which has beenreturned to the retracted position wherein the primary pivot 1 ispartially broken away to reveal actuator tube 52.

Referring now to FIG. 11 and again to FIGS. 8 and 9, there is shown anactuator embodiment which includes a non-reversing slider assembly 64slidably fitted within actuator tube 52. Like the cable connector 59(FIG. 9), the non-reversing slider assembly 64 is a means for attachingtogether the actuator spring 50, the actuator cable 51, and the threeretracting cables 54, 56, and 58, however it is additionally a means forpreventing extension of actuator spring 50. Slider assembly 64 comprisesa body 66 and a coil spring 68. Slider body 66 includes a top portionwhich is an inverted conical section joining a cylindrical section toform an annular trough 67. Fitted within this trough 67 is the coilspring 68. Upward motion of slider body 66 drives the coil spring 68down into the trough 67 and away from the inner wall surface of actuatortube 52. Downward motion of the slider body 66 causes the coil spring 68to rise and jam between the tube 52 inner surface and the conicalsurface of body 66 to prevent the downward movement of the slider. Itwill be remembered from previous description that the module wasdependent on either tension in cable 51 or external compression loads atprimary pivots l and 2 to produce tension in the three bracing cables10, 11, and 12, and that the tension in cable 51 was provided by thepreload of actuator spring 50. If the actuator spring 50 is forced toextend due to the influence of a superior external load, and the bracingcables 10, 11, and 12 go slack, the module will lose its shapestability. The nonreversing slider 64 will prevent this loss of shapestability by preventing any forced extension of actuator spring 50, andis used for those conditions where external loads are expected to exceedthe pretension of cable 51 provided by the preload of spring 50 and forother purposes which will hereinafter be described. It should be clearfrom the foregoing that the nonreversing means, comprising slider 64 andactuator tube 52, may also be utilized in the module embodiment shown inFIG. 7 to provide greater tension capability for actuator cable 51 inthose structural applications where the retraction capabilities of theembodiment shown in FIG. 8 are not required.

As previously described, stability of the module is provided by tensionin the bracing cables l0, l1, and 12. In the deployed or extendedposition the retracting cables 54, 56, and 58 are slightly slack, asshown in FIG. 8. The retracting cables become taut only during theretraction cycle. If the retracting cables are sized in length so thatthey become taut as the structure attains the fully extended positionand non-reversing slider 64 is disposed within actuator tube 52, thestructure is then in a state of static equilibrium with the tensionloads in actuator cable 51 and retraction cables 54, 56, and 58 reactedby axial compression in the six rigid link members 4, 5, 6, 7, 8, and 9.The bracing cables 10, 11, and 12 are therefore redundant in such anembodiment and may be eliminated from the structure.

Referring now to the embodiment illustrated in FIGS. 12 and 13, it willbe observed that here the actuator tube 60 is of greater length than theactuator tube 52 of FIG. 8 and extends the entire distance from pivot 1to pivot 2 when the module is in its expanded position. Three holes 61through the wall of actuator tube 60, located in alignment with thethree secondary pivots 3 in their retracted positions, are utilized asfair-leads to permit retracting cables 54, 56, and 58 to connect spring50 to the three secondary pivots 3. Expansion of the module is completedwhen the upward movement of pivot 2 is stopped by contact with the lowerend of tube 60. With the actuator tube 60 in abutting contact with pivot2, the tube 60 vbecomes a compression load carrying element of thestructure, and cable 51 within tube 60 carries tension loads up the tubeto nonreversing slider 64 wherein the tension load is transferred toactuator tube 60. A more direct method of transferring tension loadsinto tube 60 is to provide a tension latch at pivot 2. Then the actuatortube 60 becomes an integral element in the rigid triangulation of theelemental structural module, avoiding dependence on tension cable 51,which eliminates need for nonreversing slider 64, or dependence onbracing cables 10, 11, and 12 or retraction cables 54, 56, and 58 forstructural integrity.

FIG. 14 is taken at line 14-14 in FIG. 12 and shows a portion ofactuator tube 60 and primary pivot 2. Tube 60 contains a flat surface 70for abutting contact with primary pivot 2, thereby facilitating thetransfer of compression loads to the primary pivot. Also shown is anembodiment of primary pivot 2 which includes a latch means for a directtransfer of tension loads between the actuator tube 60 and the primarypivot 2. The actuator tube terminates in a probe 71 which is shaped toengage a plurality of latch pawls 72. In the embodiment shown the latchpawls 72 each have a cam surface 74 on the bottom portion which rides ona mating cam surface of latch ferrule 76 in such a manner that tensionin cable 51 causes the pawls to swing toward one another into thelatched position. A cable release embodiment is shown which is adaptedfor remote actuation, as for example by an electrical signal. Anysuitable cable release may be employed, and where the deployed structureis accessible a manual release is preferred. The cable release shownhere comprises a latch ferrule 76 fastened to actuator cable 51 by meansof a solder ball 78 and a pyro heating unit 80 surrounding the latchferrule 76. To release cable 51 and retract the structure, an electricalor mechanical impulse is impressed on the heating unit 80 which initiates the burning of the unit, producing sufficient heat to melt thesolder ball 78. Upon melting of the solder, cable 51 is drawn up intotube 60 by actuator spring 50, and latch ferrule 76 falls free. Withoutthe force of the latch ferrule acting on the latch pawls 72, theunlatching springs 82 are the dominant force, an the latch pawls moveoutward to the unlatched position. The structure thereupon is retractedby cables 54, 56, and 58 (FIG. 12) as previously described.

From the foregoing description it may be clearly seen that the discloseddouble tetragonal structure may be employed in a-variety ofconfigurations that permit a high degree of selectivity in utilizing theconfiguration which is most efficient for the intended purpose. Where noautomatic expansion or retraction is required, the structure maycomprise a plurality of modules of the type shown in FIG. 1. Where onlyautomatic expansion of the structure is required, a plurality of modulesof the type shown in FIG. 7 may be utilized, whereas if automaticexpansion and retraction are necessary, a plurality of modules of thetype shown in FIGS. 8 or 12 may be used.

Referring now to FIGS. 15 and 16, which are diagrammatic top and sideviews of a large paraboloidal structural embodiment comprised of 61individual modules, one of which is shown in detail in FIG. 17, we seean example of a curved structure utilizing modules of the typepreviously shown in FIG. 8. Such a structural embodiment utilizesmodules 'having different length rigid links of the first and secondtripods, and different cable lengths in cable assemblies and 200 to forma curved surface, whereas it will be remembered the structure of FIG. 3was comprised of equal length cables and rigid links which producedmodules having symmetry. In order to more clearly illustrate thestructural embodiment of FIGS. 15 and 16 the links have beendiagrammatically illustrated, wherein the first set of rigid links 4, 5,and 6 are shaded, the actuator tube 52 is black, cables 101 thru 106 ofcable assembly 100 are dotted lines, bracing cables 10, 11, and 12 whichform cable assembly 300 are dash lines, primary pivot 1 is a blackcircle, and primary pivot 2 is a white circle. Retracting cables 54, 56,and 58 are not diagrammatically shown in FIGS. 15 and 16 for clarity.

The structure of FIGS. 15, 16, and 17 is expandable and retractable bymeans of the expansion and retraction capability of each individualmodule in the manner previously described herein. Contiguous modules areconnected one to another in the same pattern sequence as the structuralembodiment shown in FIG. 3, such that the bracing cables 10, 1 1, and 12of cable assembly 300 define a pattern of triangles in the plan viewwherein every other triangle defines the common base of the said firsttripod (links 4, 5, and 6) and second tripod (links 7, 8 and 9) of anindividual module. Another manner of visualizing this arrangement, inrelation to FIGS. 3 and 15, is to observe that every other triangleformed by bracing cables 10, l1, and 12 of cable assembly 300 contains aprimary pivot 1 and 2 located approximately centered in the plan view ofthe triangle, one above and one below, whereas every alternate triangleof cable assembly 300 is not the base of any module tripod and does nottherefore have a primary pivot l and 2 above or below it.

It is understood that the foregoing disclosure of my invention is to beconsidered as merely illustrative and that many other arrangements maybe devised to tailor the structure to desired requirements, and that thespecification and drawings disclosed herein are not to be taken as alimitation, the spirit and scope of the invention being limited only bythe claims.

I claim:

l. A modular structure capable of being expanded and retracted whereinan individual module comprises:

a first set of three rigid links pivotally joined together at one end toform a first primary pivot;

a second set of three rigid links pivotally joined together at one endto form a second primary pivot;

the free end of each link of said first set of rigid links pivotallyjoined to the free end of a corresponding rigid link of said second setof rigid links to form three secondary pivots;

a tubular actuator having its first end pivotally attached to said firstprimary pivot and extending toward said second primary pivot; and

means located within said tubular actuator for irreversibly biasing saidfirst and second primary pivots toward each other.

2. The structure of claim 1 further comprising a plurality of retractingcables operatively connecting said secondary pivots to said irreversiblebiasing means to urge said secondary pivots toward each other.

3. The structure of claim 2 wherein the second end of said tubularactuator is adapted to structurally bear upon said second primary pivotwhen said modular structure is expanded.

4. A modular structure capable of being expanded and retracted whereinan individual module comprises:

a first set of three rigid links pivotally joined together at one end toform a first primary pivot;

a second set of three rigid links pivotally joined together at one endto form a second primary pivot;

the free end of each link of said first set of rigid links pivotallyjoined to the free end of a corresponding rigid link of said second setof rigid links to form three secondary pivots;

a tubular actuator having a first end connected to said first primarypivot and a second end adapted to structurally bear upon said secondprimary pivot when said modular structure is expanded;

a tension spring having a first end connected to said first primarypivot and located within said tubular actuator;

an extension cable connecting the second end of said tension spring tosaid second primary pivot to bias said first and second primary pivotstoward each other;

a latch for structurally securing said tubular actuator to said secondprimary pivot;

means for disengaging said latch and disconnecting said tension springfrom said second primary pivot; and

a plurality of retraction cables operatively connecting said secondarypivots to said tension spring for biasing said secondary pivots towardeach other during a portion of the travel of said tension spring.

5. A structural module, capable of being expanded and retracted,comprising:

a first set of three rigid links pivotally joined together at one end toform a first primary pivot;

a second set of three rigid links pivotally joined together at one endto form a second primary pivot;

the free end of each link of said first set of rigid links pivotallyjoined to the free end of a corresponding rigid link of said second setof rigid links to form three secondary pivots;

three flexible links, each joining together two of said secondary pivotsto complete a double tetragonal structure;

a tension spring having a first end connected to said first primarypivot;

an extension cable connected to a second end of said spring and to saidsecond primary pivot;

a plurality of retraction cables operatively connecting said secondarypivots to a second end of said tension spring for biasing said secondarypivots toward each other during a portion of the travel of said tensionspring;

a tubular actuator pivotally attached at the first end to said firstprimary pivot and extending toward said second primary pivot, saidtubular actuator containing at least a portion of said tension springand adapted to guide said retraction cables from within to outside ofsaid tubular actuator; and

means for disconnecting said tension spring from said second primarypivot.

6. The module of claim 5 wherein the second end of said tubular actuatoris adapted to structurally bear upon said second primary pivot when saidmodule is expanded.

7. The module of claim 6 further comprising a latch disposed within saidsecond primary pivot to structurally secure said tubular actuator tosaid second primary pivot.

8. The module of claim 5 further comprising irreversibility meansslidably located within said tubular actuator and operatively connectedto said tension spring to permit said spring to travel in only onedirection.

9. The module of claim 8 wherein the second end of said tubular actuatoris adapted to structurally bear upon said second primary pivot when saidmodule is expanded.

1. A modular structure capable of being expanded and retracted whereinan individual module comprises: a first set of three rigid linkspivotally joined together at one end to form a first primary pivot; asecond set of three rigid links pivotally joined together at one end toform a second primary pivot; the free end of each link of said first setof rigid links pivotally joined to the free end of a corresponding rigidlink of said second set of rigid links to form three secondary pivots; atubular actuator having its first end pivotally attached to said firstprimary pivot and extending toward said second primary pivot; and meanslocated within said tubular actuator for irreversibly biasing said firstand second primary pivots toward each other.
 2. The structure of claim 1further comprising a plurality of retracting cables operativelyconnecting said secondary pivots to said irreversible biasing means tourge said secondary pivots toward each other.
 3. The structure of claim2 wherein the second end of said tubular actuator is adapted tostructurally bear upon said second primary pivot when said modularstructure is expanded.
 4. A modular structure capable of being expandedand retracted wherein an individual module comprises: a first set ofthree rigid links pivotally joined together at one end to form a firstprimary pivot; a second set of three rigid links pivotally joinedtogether at one end to form a second primary pivot; the free end of eachlink of said first set of rigid links pivotally joined to the free endof a corresponding rigid link of said second set of rigid links to formthree secondary pivots; a tubular actuator having a first end connectedto said first primary pivot and a second end adapted to structurallybear upon said second primary pivOt when said modular structure isexpanded; a tension spring having a first end connected to said firstprimary pivot and located within said tubular actuator; an extensioncable connecting the second end of said tension spring to said secondprimary pivot to bias said first and second primary pivots toward eachother; a latch for structurally securing said tubular actuator to saidsecond primary pivot; means for disengaging said latch and disconnectingsaid tension spring from said second primary pivot; and a plurality ofretraction cables operatively connecting said secondary pivots to saidtension spring for biasing said secondary pivots toward each otherduring a portion of the travel of said tension spring.
 5. A structuralmodule, capable of being expanded and retracted, comprising: a first setof three rigid links pivotally joined together at one end to form afirst primary pivot; a second set of three rigid links pivotally joinedtogether at one end to form a second primary pivot; the free end of eachlink of said first set of rigid links pivotally joined to the free endof a corresponding rigid link of said second set of rigid links to formthree secondary pivots; three flexible links, each joining together twoof said secondary pivots to complete a double tetragonal structure; atension spring having a first end connected to said first primary pivot;an extension cable connected to a second end of said spring and to saidsecond primary pivot; a plurality of retraction cables operativelyconnecting said secondary pivots to a second end of said tension springfor biasing said secondary pivots toward each other during a portion ofthe travel of said tension spring; a tubular actuator pivotally attachedat the first end to said first primary pivot and extending toward saidsecond primary pivot, said tubular actuator containing at least aportion of said tension spring and adapted to guide said retractioncables from within to outside of said tubular actuator; and means fordisconnecting said tension spring from said second primary pivot.
 6. Themodule of claim 5 wherein the second end of said tubular actuator isadapted to structurally bear upon said second primary pivot when saidmodule is expanded.
 7. The module of claim 6 further comprising a latchdisposed within said second primary pivot to structurally secure saidtubular actuator to said second primary pivot.
 8. The module of claim 5further comprising irreversibility means slidably located within saidtubular actuator and operatively connected to said tension spring topermit said spring to travel in only one direction.
 9. The module ofclaim 8 wherein the second end of said tubular actuator is adapted tostructurally bear upon said second primary pivot when said module isexpanded.